The present invention relates to an expansion valve for a refrigerating cycle which is suitable for an air conditioning system for a motor vehicle.
Generally, the air conditioning system for the motor vehicle is constituted of the refrigerating cycle in which a refrigerant discharged from a compressor is returned to the compressor by way of a condenser, the expansion valve and an evaporator.
The expansion valve is so designed as to control super heat by evaporation temperature at an outlet of the evaporator and a pressure (low pressure) at the outlet of the evaporator. Thereby, the expansion valve is so made as to be free from being influenced by any high pressure which may cause a disturbance.
As is seen in FIG. 7, the expansion valve shows an open valve characteristic TXV which features substantially a horizontal graph. In other words, the open valve characteristic TXV is not dependent on the high pressure.
The open valve characteristic TXV of the expansion valve in FIG. 7 is obtained at an evaporator outlet temperature Te of 0xc2x0 C. In FIG. 7, the ordinate is an evaporator outlet pressure Pe (more specifically, Peout), while the abscissa is a discharge pressure Pd. There is defined an expansion valve closed zone on substantially an upper side of the open valve characteristic TXV. Contrary to this, there is defined an expansion valve open zone on substantially a lower side of the open valve characteristic TXV. Moreover, as is seen in FIG. 7, on substantially a left side and substantially a right side of an operation area B, there are defined, respectively, a first non-operation area A-1 and a second non-operation area A-2, in each of which the compressor does not operate.
Furthermore, FIG. 7 shows a control valve characteristic C/V featuring a downward slope in accordance with the discharge pressure Pd which gets higher gradually. The control valve characteristic C/V is the one that is obtained when a variable displacement compressor equipped with a control valve is combined with the expansion valve that is set at the open valve characteristic TXV featuring the substantially horizontal graph. In the expansion valve closed zone, there is caused an interfered control area xe2x80x9cdxe2x80x9d as is depicted by diagonal lines in FIG. 7.
Failures such as hunting and the like may occur, for example, under the following three conditions combined: 1. The expansion valve is set at 5 kg/cm2G (namely, in the interfered control area xe2x80x9cdxe2x80x9d). 2. Being in the expansion valve closed zone. 3. The compressor is likely to pull the refrigerant forcibly.
In order to prevent such failures from occurring, there is provided one possible solution, that is, to make the control valve characteristic C/V non-linear so that the interfered control area xe2x80x9cdxe2x80x9d does not occur up to an intersection point P. However, such solution complicates the constitution of the control valve characteristic C/V.
For preventing, with ease, the interfered control area xe2x80x9cdxe2x80x9d from occurring, the open valve characteristic TXV of the expansion valve is set higher than the control valve characteristic C/V, as is depicted by a dashed line in FIG. 7. In this case, however, the valve opening of the expansion valve gets large when the discharge pressure is high (high load applied) since the control valve characteristic C/V is included in the expansion valve open zone.
When the valve opening of the expansion valve gets large, the refrigerant flowrate will increase. Thereby, it becomes impossible to take the proper super heat, and the refrigerating power is deteriorated. Simultaneously with this, consumption (power) of the variable displacement compressor is increased, thus ending up an increase in cost (not preferable).
It is, therefore, an object of the present invention to provide an expansion valve for a refrigerating cycle for solving the above mentioned problems.
According to a first aspect of the present invention, there is provided a refrigerating cycle comprising a condenser, an evaporator, a variable displacement compressor, and an expansion valve. The variable displacement compressor is connected between the condenser and the evaporator to form the refrigerating cycle, and comprises a control valve designed to provide a control valve characteristic sloping downward with increase in a discharge pressure. The expansion valve is connected between the condenser and the evaporator, to return a refrigerant from the condenser through the evaporator to the variable displacement compressor, and designed to provide an open valve characteristic whose slope is approximately equal to the downward slope of the control valve characteristic.
According to one of a second aspect and a third aspect of the present invention, there is provided a refrigerating cycle comprising a condenser, an evaporator, a variable displacement compressor, and an expansion valve. The variable displacement compressor is connected between the condenser and the evaporator to form the refrigerating cycle, and comprises a control valve designed to provide a control valve characteristic sloping downward with increase in a discharge pressure. Furthermore, the variable displacement compressor is adapted to operate in an operation area defined between a first non-operation area and a second non-operation area opposite to the first non-operation area, the second non-operation area defining the discharge pressure higher than the discharge pressure that is defined in the operation area. The expansion valve is connected between the condenser and the evaporator, to return a refrigerant from the condenser through the evaporator to the variable displacement compressor, and designed to provide an open valve characteristic which intersects, at an intersection point defined in one of the first non-operation area and the second non-operation area of the variable displacement compressor, with the control valve characteristic of the control valve.